The present invention is a new structural member that has its greatest applicability as a column, as underlying supports for a plate, or as a beam.
Parabolic or catenary suspensions are very effective loading structures. Large uniform forces can be supported depending on the cable tension and the tensile strength of the cable. In an ordinary beam or plate, a suspension is not achieved because the beam or plate is not in tension. If one could artificially induce a tension in a beam or plate, a load could be supported as effectively as with a suspension and better than with a freely supported beam or plate. The structure necessary to anchor and to support a cable normally preclude using a suspension except for certain larger applications such as bridges. If tension can be applied without the complex structure of, for example, a suspension bridge, beams and panels could support greater loads with less deflection. It is an object of the present invention to disclose and provide a structure for artificially inducing the tension in a beam or panel so that they could be as effective as suspension supports.
Most metals have a maximum stress in compression that is approximately equal to the maximum stress in tension. In compression, however, if the length of the member is greater than 10 to 15 times the smallest cross-sectional dimension, the member is considered a column. Columns fail because they buckle or laterally deflect, which is caused by minor variations in the column, both in shape or in material homogeneity and by loads that are aligned incorrectly. The long moment arm of a column allows small variations to cause failure in a loaded column. A discussion of the mechanics of columns is set forth in Tool Engineers Handbook, 2d Ed., p. 101-29 (1959).
Euler discovered the mathematics of a column. He defined a quantity called the slenderness ratio as the length of the column divided by the smallest radius of gyration; L/k, where L=length and k=minimum radius of gyration. Euler determined experimentally that where the slenderness ratio is less than about 30 the column may be considered to be an ordinary compression member. Where the slenderness ratio is greater than 30, however, the effects of lateral deflection increase. Thus, if the slenderness ratio is below 30, the member can support a rather constant load as a function of the slenderness ratio, but at higher ratios, the maximum supportable load decreases as a function of the slenderness ratio.
In some applications the slenderness ratio may be decreased by increasing the diameter of the column, but the additional material adds cost and weight. In applications such as aerospace where weight is a significant factor, size reengineering may be impractical. It may also be possible to better fix the ends of the column to decrease the effective column length, and it may also be possible to restrain the midsection of a column thereby splitting the effective column length into shorter beams. Such solutions may be impractical for the intended environment.
The patent literature discloses a number of attempted solutions to this problem. For example, Meckler, U.S. Pat. No. 3,538,653 (1970) employs hydraulics to counteract some compression forces. Werth, U.S. Pat. No. 2,857,755 (1958), Abbott, U.S. Pat. No. 3,167,822 (1965) and Rieve, U.S. Pat. No. 3,516,211 (1970) disclose systems for prestressing concrete or prestressing the reinforcing rods. Hollander, U.S. Pat. No. 3,232,628 (1968) shows a prestressed tube.
One of the objects, therefore, of the present invention is to provide a column that can resist buckling even though the slenderness ratio is above 30, even substantially above 30. This helps meet an ultimate goal of providing a low cost, low weight structural member of surprising resistance to compression and buckling under what would be considered an excessive load for a column.
A further object of the present invention is to be able to take advantage of high tensile strength fiber material such as aramid fiber in a compression member even though the fiber is not rigid. It is also an object of the present invention to use such materials by converting the compressive forces into unidirectional tensile forces.
Conventional beams also have loading limits whether supported at both ends or cantilevered. For this discussion, beams are considered to be structural members such as long narrow members and long and wide plates subjected to transverse loading. In a beam, the moment is resisted jointly by tensile and compressive stresses along opposite surfaces of the beam. If at the surfaces the yield point of the material is exceeded, the beam fails.
As in the case of columns, the maximum transverse load that a beam can support without exceeding the yield point depends on the material and its configuration. For a given material and shape, increasing the thickness of the material will increase the load that a beam can support. It may be impractical, however, to change the thickness because of space or weight considerations.
A beam need not fail (i.e. the beam material need not yield) for a particular beam application to be unsatisfactory. A beam may deflect too much under load for it to be practical. The deflection depends in part on the modulus of elasticity of a material. Once the material is chosen and the configuration set, the deflection is fixed. For example, assume there is a 50 foot (15.2 m) long steel tube with a 0.82 in (21 mm) outside diameter freely supported at its ends. One would expect by calculation that it will sag at its midsection approximately 7.5 feet (2.3 m) due to its own weight. Decreasing the deflection would allow the beam to be used in many applications especially if the deflection were less than an inch. Sometimes, even for relatively short spans, small deflections are unacceptable, and stiffer supports may be necessary where a supported member must be ridgidly secured.
In certain applications where high stiffness to weight ratios are important, composites or sandwiches of aluminum and carbon, boron or silicon carbide are being used. These systems depend on the high modulus of elasticity of carbon and boron. Although better than most metals, they have a limit in their ability to reduce deflection because their modulus of elasticity is at most doubled.
It is an object of the present invention, therefore, to disclose and provide a lightweight and compact structural member capable of supporting heavier loads than heretofore thought possible.
As will be seen from the descriptions of the invention, some of the elements of the present invention will be subjected to very high pressure. In one embodiment, for example, a piston is mounted in a tube filled with fluid under extremely high pressure. The pressure will cause the inside diameter of the tube to increase as an inverse function to the modulus of elasticity of the tube material. One of the objects of the present invention, therefore, is to disclose and provide a piston-tube arrangement which effectively seals the space between the tube and piston even when the tube expands from hydraulic pressure.
It will also be shown that there is some deflection, even though small, in the beam of the present invention. One of the objects of the present invention is to further compensate for this small deflection by so arranging the parts so that the deflection may be induced in one direction perpendicular to the axis opposite the direction of the intended load. For example, an upward bow could be induced so that when the structural member was mounted horizontally, it would resist downward loads. Another object is to use two of the pre-bowed members generally parallel to each other, which could be rotated about their axis to adjust the bowing preload so that after loading the beam would be flat.